Nuclear reactors are typically in the form of a boiling water reactor having suitable nuclear fuel disposed in a reactor pressure vessel in which water is heated. The water and steam are carried through various components and piping which are typically formed of stainless steel, with other materials such as alloy 182 weld metal and alloy 600 being used for various components directly inside the reactor pressure vessel.
Materials in the reactor core region are susceptible to irradiation assisted stress corrosion cracking. This is because the material in the core region is exposed to the highly oxidizing species generated by the radiolysis of water by both gamma and neutron radiation under normal water chemistry conditions, in addition to the effect of direct radiation assisted stress corrosion cracking. The oxidizing species increases the electrochemical corrosion potential of the material which in turn increases its propensity to undergo intergranular stress corrosion cracking or irradiation assisted stress corrosion cracking.
Suppression of the oxidizing species carried within such materials is desirable in controlling intergranular stress corrosion cracking. An effective method of suppressing the oxidizing species coming into contact with the material is to inject hydrogen into the reactor water via the feedwater system so that recombination of the oxidants with hydrogen occurs within the reactor circuit.
This method is called hydrogen water chemistry and is widely practiced for mitigating intergranular stress corrosion cracking of materials in boiling water reactors. When hydrogen water chemistry is practiced in a boiling water reactor, the electrochemical corrosion potential of the stainless steel material decreases from a positive value generally in the range of 0.050 to 0.200 V (SHE) under normal water chemistry to a value less than xe2x88x920.230 V (SHE), where SHE stands for the Standard Hydrogen Electrode potential. When the electrochemical corrosion potential is below this negative value, intergranular stress corrosion cracking of stainless steel can be mitigated and its initiation can be prevented.
Considerable efforts have been made in the past decade to develop reliable electrochemical corrosion potential sensors to be used as reference electrodes which can be used to determine the electrochemical corrosion potential of operating surfaces of components.
The typical electrochemical corrosion potential sensor experiences a severe environment in view of the temperature of the water well exceeding 88xc2x0 C.; relatively high flow rates of the water up to and exceeding several m/s; and the high nuclear radiation in the core region.
A drawback of currently available sensors is that they have a limited lifetime in that some have failed after only three months of use while a few have shown evidence of operation for approximately six to nine months. Two major modes of sensor failure have been the cracking and corrosive attack in a ceramic-to-metal braze used at the sensing tip, and the dissolution of a sapphire insulating ceramic material used to electrically isolate the sensing tip from the metal conductor cable for platinum and stainless steel type sensors.
Accordingly, it is desired to improve electrochemical corrosion potential sensors in terms of durability and longevity.
A ceramic sensor is provided that includes a ceramic tube having a closed end and an open end, and a metal sleeve having open first and second ends. The ceramic tube is formed of stabilized zirconia. The metal sleeve extends about the open end of the ceramic tube and is sealingly joined thereto at a contact region by a brazeless bond resulting from hot isostatic pressing. A mixture of metal and metal oxide powders is provided within the ceramic tube adjacent the closed end, and a conducting wire extends from the mixture of metal and metal oxide powders to the second end of the metal sleeve.
In another embodiment of the present invention, a method for providing a ceramic sensor calls for inserting the ceramic tube into the metal sleeve, and executing a hot isostatic pressing step, in an environment and at a temperature and a pressure sufficient to form a sealed joint therebetween at said contact region to form a brazeless casing. The ceramic sensor is then completed.